We just got back from the beach a few weeks ago. My dd is 21 months old, so she's kind of in between your kids' ages. The best thing we brought was a "mini pool." It is a tiny, inflatable wading pool like this
. That way, dd had some water to splash in, but was at a safe distance from the ocean, when DH and I wanted to sit and relax. It only took about 2 buckets to fill it. We also brought along a small boat to float in it. Between that and a shovel, DD was happy as a clam.
Don't forget to reapply sunblock every 2 hours if your kids play in water. Water, sweat and sand causes it to wear off quickly.
A few new, small toys and books made our car travel time much easier. They were also a big help during waiting time at restaraunts.